1. Field of the Invention.
The invention relates to the field of metal-oxide-semiconductor (MOS) buffers.
2. Prior Art.
There are numerous circuits used in MOS integrated circuits for input buffering. In a typical application, a buffer receives an input signal such as address, data or control signals, and converts it from the input level to an MOS level. Most commonly, TTL level signals (e.g., 0.8 and 2.0 volts) are converted to MOS level signals (e.g., 0 and 5 volts). Other conversions are also used such as from ECL to MOS levels.
The characteristics of MOS circuits vary greatly with temperature. MOS circuit characteristics also vary within a given process, and indeed, two die spaced-apart on the same wafer can have transistors with considerably different threshold voltages. For some applications, circuits must operate over a wide temperature range such as -55 degrees C. to 125 degrees C. In fact, the internal temperature of the integrated circuit may be as high as 155 degrees C. when the circuit operates in an environment of 125 degrees C. Under these stressful temperatures, the circuit must also operate with power supply variations in the range of 10%. Buffers as well as other MOS circuits in some applications must be designed to operate over this wide temperature range. This is difficult to achieve particularly because of the process variations.
Most MOS buffers use inverters in a variety of arrangements. These buffers have poor temperature and process variation characteristics. Other MOS buffers use a differential sensing arrangement where a reference potential is compared to the input signal. These buffers have substantially better temperature characteristics; however, they are relatively slow and consume considerable power. Prior art MOS buffers are shown in U.S. Pat. Nos. 4,048,518 and 4,437,171; ISSCC81, Digest of Technical Papers, pages 17 and 21; and ISSCC82, Digest of Technical Papers, pages 249 and 257.
In a search conducted for Applicant on the "feedback" arrangement used in the present invention, three patents were uncovered. U.S. Pat. No. 4,199,693 is of only general interest. U.S. Pat. No. 4,430,582 discloses a buffer circuit with feedback and a compensated reference potential. One disadvantage to this circuit is that current is drawn from the compensated reference potential circuit, unlike the present invention. U.S. Pat. No. 4,410,813 discloses a different circuit arrangement which Applicant believes is not as responsive to environmental changes as the present invention.
As will be seen, the present invention discloses an MOS input buffer which is substantially insensitive to temperature variations and processing variations. In the preferred embodiment, the buffer operates successfully over a temperature range of -55 degrees C. to 155 degrees C. The invented buffer is relatively fast and of low power.